1. Field of the Invention
This invention relates to a linear polyester. More especially, this invention relates to a coating composition comprising a specific linear polyester and an aminoplast resin. This invention is particularly concerned with coating compositions of linear polyesters containing moieties of terephthalic acid which are soluble without turbidity and are solid at room temperature which coating compositions further contain aminoplastic resins. The coating compositions of this invention can further contain pigments and conventional varnish adjuvants. Especially contemplated is a coating composition containing 10 to 25 weight percent of an aminoplastic resin and 90 to 75 weight percent of a linear polyester containing hydroxyl groups further characterized in that its acid component contains 45-55 mole percent of a residue of terephthalic acid or esterifiable derivative; 35 to 45 mole percent of a residue of another aromatic dicarboxylic acid or its esterifiable derivative and up to 20 mole percent of a cycloaliphatic and/or aliphatic dicarboxylic acid, the linear polyester residue further being characterized in that its alcohol component comprises 20 to 30 mole percent of a residue of hydroxypivalic acid neopentyl glycol ester, 20 to 30 mole percent of a residue of an aliphatic diol with a branched carbon chain and 40 to 60 mole percent of a residue of ethylene glycol (of which up to 10 percent can be replaced by ether diols or by diprimary aliphatic diols or more than two carbon atoms in the chain).
2. Discussion of the Prior Art
From German Auslegeschrift No. 1,805,182 it is known to use linear saturated polyester resins as binding agents for bake-on varnishes, wherein polyester resins containing hydroxyl groups and having average molecular weights between 600 and 3000, prepared from dicarboxylic acids such as phthalic acid or hexahydrophthalic acid, for example, and various diols, especially primary-secondary bivalent alcohols such as propanediol-1,2, are hardened with melamineformaldehyde resins by crosslinking.
The varnish films thus obtained, however, do not have the especially high elasticity combined with high surface hardness required in the coating of sheet metals (Comparative Experiments 1 and 2). According to the quality indexes of the European Coil Coating Association (ECCA), a test known as the T-bend test is used to determine the ability of the material to be bent 180.degree. around mandrels of various diameters. The following formula is used: ##STR1## The smallest mandrel diameter is the diameter at which no cracks reaching all the way to the metal are found when the bend zone is examined with a ten-power magnifier. In Category 2, the minimum required value of T is 2.
As Comparative Experiments 1 and 2 will show, this requirement is not satisfied by lacquers of the Examples given in German Auslegeschrift No. 1,805,182. In addition, the polyesters described therein are highly viscous fluid substances at room temperature.
German Offenlegungsschrift No. 2,211,059 discloses the preparation of highly elastic coatings, in which the binding agents are polycondensation products of dimethyl terephthalate and diol mixtures with 5 to 50 mole-% of hydroxypivalic acid neopentyl glycol ester (HPN). Polyesters prepared in accordance with the Examples are solid substances at room temperature, but they do not dissolve clear in such commonly used solvents as, for example, a mixture of Solvesso 150 and ethyl glycol acetate in a ratio of 9 to 1 parts by weight (See Comparative Experiments 3 and 4), and in addition they acquire a dark yellow to brown discoloration.
Varnish films which contain these polycondensation products, in accordance with the Examples, as the sole binding agent, are not resistant to conventional lacquer solvents. In addition, as much as 35 weight-percent of solvent is needed to obtain a varnish with a viscosity between 100 and 120 seconds per DIN 53,211 that can be used in the coil coating process.
It is an object of this invention, therefore, to provide high quality coating substances of obtimum suitability for the coil coating process having a high solids content, in which the polyester resin on which they are based can readily be handled, is solid as room temperature and has a high solubility and a limited selection of special high boiling solvents suitable for the coil coating process.
It is another object of the this invention to provide polyester resins suitable for these coating substances which can be readily packed, shipped and processed in solid form e.g. in the form of lumps packed in bags.
It is a further object of this invention to provide a process for coating a sheet metal substrate with a cross-linkable coating composition.